1. Field of the Invention
The present invention relates to telephoto optical lens systems and more particularly to an improved simplified focusing mode of operation.
2. Description of the Prior Art
In the general field of photographic lens systems, focusing has been frequently accomplished by the shifting of the entire lens element assembly. This is sometimes a relatively easy expedient for focusing in smaller compact lens assemblies of a fixed focal length.
However, in a telephoto lens assembly, its diameter, overall length and weight are by necessity increased over both a standard and wide angle lens systems. Accordingly, any focusing arrangement which requires the shifting of the entire telephoto lens system would also require mechanisms that are by necessity relatively complex, strong and durable. Additionally, the shifting distance for focusing is also necessarily increased. Needless to say these requirements all contribute to a higher cost for the lens barrel and produce both increased weight and size.
Modern photographic demands have required more compact and lighter telephoto lens systems. As any photographer knows, the relative weight of a telephoto lens system makes this system extremely hard to be held stable and adds human physiological vibrations that are introduced into the image plane of the camera.
Optical designers, when working with a telephoto lens assembly design have attempted to simplify focusing by shifting a rear lens component. These rear lens components are generally relatively small in diameter and of light weight, especially compared with the more conventional systems wherein the entire lens assembly are shifted as a unit. By utilizing this design approach, a shiftable rear lens component in a telephoto lens system generally has the advantage of a reduced lens movement, less barrel manufacturing cost and greater ease of operation. This design approach, however, suffers a disadvantage in that a variation in aberrations, due to movement of this rear lens component, are greater than in the conventional system. Particularly, excessive negative spherical aberration is introduced into the system. As is well-known in the optical design field, a telephoto lens system in its nature requires a considerable under correction of spherical aberration in the front lens components. This under balance is remedied by over correction of the spherical aberration in the rear lens components. This balance is disturbed when the rear lens component is shifted, thus introducing an unwanted negative spherical aberration.
Another factor in designing a focusing mode of operation for a telephoto lens system relates to the paraxial region and the particular direction of movement of the shiftable lens components in the lens system when focusing at an object which is relatively close to the front of the lens barrel. In this regard, consideration must be given to the image formation characteristics of the shiftable lens components. Thus, in a paraxial image formation diagram of a lens system, the lens system will be focused at a close object when the movable focusing lens components are shifted in the direction of diminishing the distance between the object point and the image point for the shifting lens components. Supposing that the rear lens group of a telephoto lens system is divided into a plurality of lens components, focusing is generally possible unless the object distance of each lens component in the rear lens group, is shorter than the image distance thereof. Generally, focusing at a relative close object is made possible when one of the divided rear lens components is shifted toward the object side and has a positive refractive power or is shifted toward the image side if it has a negative refractive power.
A system of focusing a telephoto lens assembly at relatively close objects by shifting a positively refracting rear lens component has a disadvantage in that the correction of aberrations is made difficult by the necessary increased negative refracting power of the other lens components in the rear lens group. If, on the other hand, the negative refractive power is designed into the other lens components in the rear lens group to permit a correction of spherical aberration, then it becomes extremely difficult to obtain the desired telephoto ratio. For these reasons, the prior art, when seeking a simplified focusing mechanism, has resorted to shifting a negative refracting lens group toward the image side. Typical examples of such focusing systems are described in the following Japanese Patent applications that have been laid open to inspection to the public, Japanese Patent Application Laid Open No. 17723/1974; Japanese Patent Application Laid Open No. 139732/1975 and Japanese Patent Application Laid Open No. 32327/1976.
Thus, there is still a demand in the prior art to provide a simplified mode of focusing in telephoto lens assemblies.